In terms of realistic / photographic portrait drawing:
Contour drawing is the foundation of many successful portrait (even still life) drawing. When you miss the correct proportion as early as the 'guide' lines, you may not love a client's reaction to your finished work.
I am fascinated with a sketch that resembles the subject --i would frame it all the way, forgetting the realistic aspect, as it is already a powerful artwork itself. But not all sketches come like that. Hence, I would push more practice time on sketching / contour drawing skill --color, texture, and value rendering come next.
The Gentle Giant exercise offers ample opportunities to improve one's skills in line drawing. I like it.
~papyVir